Device for determining the condition of transformers



Jan. 27, 1931. 5 RUM]: 1,790,024,

DEVICE FOR DETERMINING THE CONDITION OF TRANSFORMERS Filed March 2, 1926Patented Jan. 27, 1931 UNITED STATES PATENT-O FICE SIGURD BUMP, FBOSTON, MASSACHUSETTS, ASSIGNOR TO ALMIERICAN BROWN BOVERI ELECTRICCORPORATION, OF NEW YORK,.N. Y., A CORPO RATION DEVICE FOR DETERMININGTHE CONDITION OF TRANSFORMERS Application filed March 2, 1926. SerialNo. 91,768.

In operating transformers of large output it is important that internaldamages, which may occur, and which are generally invisible, andtherefore are not noticed for long periods of time, may be determined asearly as possible in order that they may be removed. Such damages,especially short circuits between core laminations and other defects ininsulation, increase in time and lead to sudden serious overheating andburning, sometimes to fire and to complete uselessness of the wholeapparatus.

Successful use has been made of the gas development which is due tolocal heating and forming of arcs on the damaged part in order todetermine defects. Such gas development, however, occurs only when theheating is sufficient to evaporate or decompose a certain amount of oil.

The present arrangement for protection, however, acts earlier andindicates the defect in a station where a gas development has notstarted. The method is based on the invariable appearance of watt lossesin a damaged part. Such losses must certainly be balanced by the networkand they are always balanced only by one side, that is, either by thehigh tension side or by the low tension side of the trans former. Thecurrent of both trans-tori or sides compared and referred to a ratio oftransforn'iation 1, results normally in a differential current which isequal to a certain watt loss, namely, the iron loss plus copper loss,which is caused by the magnetizable current.

This watt loss is constant as long as the pnmary voltage of thetransformer is constant, therefore, the differential current equals thewatt loss. The differential current or differential output has alreadybeen used for indicating defects in transformers as well as foroperating safety devices as soon as such current or output exceed theirnormal values. Each watt consumption resulting from any losses can,theoretically, be replaced by a resistance which would cause the samelosses. Such theoretically compensation resistance although practicallynot present can be measured with an ohmmeter. According to theinvention, an ohmmeter really will therefect.

fore be used for indicating a transformer de- The drawingdiagrammatically shows one application form of the invention.

a indicates the low tension winding and b the high tension winding ofthe transformer: to be pro- .tected. On both sides are connected currenttransformers c and d, the secondary windings of which are connectedwinopposition and are arranged in the circuit of an ohmmeter e, which isconstructed in accordance with the cross coil principle. The stationarycoil f and coil 9 of the movable system, which are connected in series,are traversed by the differential current. Coil h of the movable relaysystem'lies at a constant .angle to coil 9 and is fedby thev voltagetransformer z, the, primary coil of which is connected to the lowtension.side of the transformer. By the action of the ohmmeter arm mconnected to the movable system, a

contact is of an auxiliary circuit, diagrammatlcally shown inthedrawing, 1S operated,

ing, alarm or protection apparatus.

which auxiliary circuit contains the indicat- It Will be understood fromthe foregoing that the movable system of ohmmeter e coniprises coils g.and h and the arm m. these parts being fixedly connected hfigetlu-r tomove as a unit with relation to coil The ohmmeter is so (-(mstructedthat the arm on operates contact k in the c'vcntthc con'ipensationresistance increases a certain amount over the normal no-load losses ofTherefore the connections can be adjusted so that the arrangement willbe sensltive to a very small excess over the allowable losses.

the resistance. f

The new arrangement may also be made other apparatus and electricmachines.

with certain modifications for protection of all events, it can be usedwherever internal damage causes a change of the resistance value of thewinding to be protected.

I claim:

1. The combination of a transformer having primary and secondary leadconnections, of a protective system therefor comprising a pair ofdifferentially-connected transformer windings associated with theprimary and secondary lead connections, and a relaydevice in series withsaid differential transformers energized by the differential effect ofthe currents of said windings to operate only in response to thevariations of-the watt component of said differential current.

2. In electrical protective systems for transformers comprising a pairof serially connected transformer windings, differentially energized bythe primary and secondary currents of the transformer to be protected, arelay device having a pair of movable coils fixedly connected and astationary coil cooperating there-With, one of said movablecoils andsaid stationary coil being connected in series with saidserially-connected transformer windings, the other of said movable coilsbeing energized by a current proportional to the voltage across thetrans former to be protected to actuate said relay in response to theWatt component of the differential current in said serially-connectedtransformer windings.

'3. In a protectlve system, a transformer having primary leadconnections and second- 'ary lead connections, a current transformer inthe primary lead connections, a current transformer in the secondarylead connections, an ohm-meter relay comprising a stationary coil and apair of movable coils, and said movable coils being fixedly connectedand differentially actuated by said stationary coil, the secondarywindings of said current transformers being differentially connected inseries with the serially.v connected stationary coil and one movablecoil of said relay, the other of said n'lovablc coils being suppliedfro-m across said transformer.

all

4. In a protective system, a main transformer, a pair of secondarytransformers respectively connected to the windings of said maintransformer, the secondary windings of said secondary transformers beingdifferentially connected in series, an ohmmeter in series with saiddifferentially connected transformers,and a transformer connected acrossthe primary of said main transform-er,the secondary of said lastmentioned transformer being connected in series with the voltage coil ofsaid ohmmeter.v

In testimony whereof I hereunto sign my name.

SIGURD BUMP.

